you can buy a new exhaust, intake, chip, pulley's, intake manfold, just to name a few,
or you could buy a aa supercharger for 5k then have a little left over for something else maybe exhaust too, a supercharger would definetly give you more power, i know another good thing is to like buy a new rear axle or differential or something not sure
you should really contact your local bmw tuner and aks them they could hook you up and include installation fees
also check out
m3motorwerks Active autowerks these guys are in miam i don't know if that's close to you or not, i used to live in ft. lauderdale when i was 5
if you live near tampa these guys are aa distributors
Cars & Concepts
Tampa, FL
813-348-6183
Thomas Boylan
that supercharger would give you 355+ hp at the crank and like 285 at the rear wheels that is nizice